(1) A new Mg2+-dependent phosphoseryl/threonyl protein phosphatase from bovine brain has been identified as a 78,000 molecular weight monomeric protein. Substrate specificity and inhibitor susceptibility establish that the enzyme is different from any other known protein phosphatase. Another protein of the same molecular size copurifies with the phosphatase and, when phosphorylated by protein kinase C or casein kinase II, serves as an excellent substrate for the phosphatase. Amino acid sequence and composition analyses indicate that this substrate is a homolog of a human leukemogenic protein called SET. The phosphatase may play an important role in cell differentiation and proliferation. (2) Cytosolic calcium oscillation in HeLa cells has been studied. Electroporation, a method by which plasma membranes are opened by the application of a suitable level of voltage to permit entry of various substances, proves to be a very useful tool. This approach makes it feasible to follow the changes of different cellular components as a function of time in a population of cells. Introduction of a calmodulin antagonist, W-7, and a calmodulin-independent type II kinase catalytic domain into HeLa cells revealed that activation of the endoplasmic reticulum ATPase calcium pump by this kinase may be a required step in calcium oscillation. Phosphorylation of the inositol tris phosphate receptor by this enzyme also may occur.